1. Field of the Invention
The present invention relates to a glass assembly cutting method, a package manufacturing method, a package, a piezoelectric vibrator, and an oscillator, an electronic device, and a radio-controlled timepiece.
2. Description of the Related Art
Recently, a piezoelectric vibrator (package) utilizing quartz or the like has been used in a cellular phone and a portable information terminal as the time source, the timing source of a control signal, a reference signal source, and the like. The piezoelectric vibrator of this type is proposed in a variety of forms, and an SMD-type piezoelectric vibrator is one example. The SMD-type piezoelectric vibrator includes, for example, a base board and a lid board which are bonded to each other, a cavity formed between the two boards, and a piezoelectric vibrating reed (electronic component) accommodated in a state of being airtightly sealed in the cavity.
When the piezoelectric vibrator is manufactured, recess portions for cavities are formed on a lid board wafer, and a piezoelectric vibrating reed is mounted on a base board wafer. Thereafter, the two wafers are anodically bonded with a bonding layer (bonding material) disposed therebetween, thus obtaining a wafer assembly in which a plurality of packages is formed in the matrix direction of the wafers. Then, the wafer assembly is cut into respective packages (cavities) formed on the wafer assembly, whereby a plurality of piezoelectric vibrators in which the piezoelectric vibrating reed is airtightly sealed in the cavity is manufactured.
As a method of cutting the wafer assembly, there is known a method in which the wafer assembly is cut (diced) along its thickness direction using a blade having diamond attached to its tooth tip, for example.
However, the blade cutting method has the following problems. Since it is necessary to provide a cutting zone between formation regions of the piezoelectric vibrators considering the width dimension of the blade, the number of piezoelectric vibrators obtainable from one wafer assembly is small. In addition, since the blade cutting method produces chippings during the cutting, the wafer assembly is likely to start breaking with the chippings, and the cutting surface is coarse. Moreover, another problem is poor production efficiency due to its low cutting speed.
There is known another cutting method in which a cut (scribe line) is inscribed along an intended cutting line on the surface of the wafer assembly using diamond buried in a tip end of a metal rod, and the wafer assembly is cut by applying a breaking stress along the scribe line.
However, since this cutting method produces many chippings on the scribe line, there is a problem in that the wafer assembly is likely to start breaking with the chippings, and the cutting surface is coarse. Therefore, it is difficult to use this method in manufacturing such small electronic components as the piezoelectric vibrators.
In order to solve the above-mentioned problems, Japanese Patent 3577492, for example, discloses a method of performing a chemical treatment on a wafer assembly so as to remove chippings produced when a scribe line is formed and applying a mechanical or thermal stress to the scribe line to cut and divide the wafer assembly.
However, the chemical treatment requires a special chemical treatment machine, and it is necessary to perform a post-treatment on the wafer assembly after the chemical treatment. Therefore, there is a problem in that the equipment cost and the number of manufacturing steps are increased.
In addition, JP-A-2000-219528, for example, discloses a method of irradiating a laser beam onto an intended cutting line of a wafer assembly to form a scribe line and rapidly cooling the wafer assembly, thereby cutting the wafer assembly using a thermal shock generated during the rapid cooling.
However, since the method of JP-A-2000-219528 involves rapidly cooling the wafer assembly, there is a concern about deformation on the cutting surface. Therefore, this method is not suitable for cutting such small electronic components particularly as piezoelectric vibrators.
In recent years, there has been developed a method of forming a groove (scribe line) by irradiating a laser beam onto one surface of the wafer assembly and bringing a cutting blade into contact with the other surface of the wafer assembly, thus cutting the wafer assembly by applying a mechanical breaking stress with the cutting blade. By the contacting cutting blade, the breaking stress is concentrated on the wafer assembly. In this way, a crack is formed at the groove in a thickness direction of the wafer assembly. As a result, since the wafer assembly is cut in such a way that it is divided along the groove, the wafer assembly can be smoothly cut along the intended cutting line.
When the scribe line is formed by laser irradiation, the width dimension thereof is determined by the spot diameter of the laser beam, and the depth dimension is determined by the laser irradiation time (the amount of energy applied to the wafer assembly).
In this case, if the depth dimension of the scribe line is too much larger than the width dimension, the amount of laser energy applied to the wafer assembly becomes too large, and thus the wafer assembly will be damaged greatly. As a result, the wafer assembly may be destroyed (crashed) in pieces when the breaking stress is applied during the cutting. Moreover, there is a problem in that the laser irradiation time increases, and thus the manufacturing efficiency decreases.
On the other hand, if the depth dimension of the scribe line is too much smaller than the width dimension, there is a problem in that the scribe line does not serve as the breaking starting point at the time of applying the breaking stress, and the wafer assembly is hardly broken.
In addition, when a wafer assembly 203 shown in FIG. 19 is cut, since the wafer assembly 203 includes two board wafers 200 and 201 which are laminated with a bonding layer 202 disposed therebetween, it is necessary to apply a larger breaking stress to the wafer assembly 203 compared to the case of cutting a single board wafer.
In this case, when the breaking stress is applied to the wafer assembly 203 using a cutting blade 205 having a small blade edge angle and a small thickness, the cutting blade 205 may be bent. Therefore, the blade edge comes into contact with a position different from the scribe line M′, and the breaking stress is concentrated on that position. As a result, there is a problem in that the wafer assembly 203 starts breaking from the position different from the scribe line M′ (namely, a so-called slip phenomenon occurs). Therefore, the wafer assembly 203 starts breaking obliquely from that position. On the other hand, if the blade edge angle of the cutting blade 205 is too large, even when the cutting blade 205 is brought into contact, the breaking stress is not concentrated on the wafer assembly 203, and the wafer assembly 203 may be destroyed (crashed) in pieces.
As a result, in the worst case, the cavity C may communicate with the outside, thus making it unable to maintain the airtightness in the cavity C. Such a product will be treated as defective, there is a problem in that the number of non-defective products picked out of the wafer assembly 203 will decrease and the yield will decrease.